Posterior stabilized knee prosthesis

ABSTRACT

A knee joint prosthesis includes a femoral component having an anterior side, a posterior side, a pair of laterally spaced condylar portions, and an intercondylar portion joining the condylar portions and including a recess. A cam surface is located adjacent the intercondylar recess on the anterior side of the femoral component and has a saddle shape that is defined by an at least substantially concave first radius of curvature and a convex third radius of curvature that is perpendicular to the concave first radius of curvature. The prosthesis also includes a tibial component including a platform having an upper surface that includes first and second laterally spaced concavities. Each concavity is adapted for receiving one condylar portion of the femoral component. The tibial component has a tibial post for reception in the intercondylar recess of the femoral component. The tibial post has a saddle shaped anterior cam surface that is complementary to the saddle shaped anterior cam surface of the femoral component and is defined by an at least substantially convex second radius of curvature and a concave fourth radius of curvature that is perpendicular to the convex second radius of curvature.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. provisional patentapplication Ser. No. 60/826,844, filed Sep. 25, 2006, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to joint replacement surgery andmore particularly, to a posterior stabilized knee prosthesis thatincludes a modified femoral anterior cam surface and a modified anteriorface of a stabilizing post of a tibial insert that results in areduction of stresses at these surfaces and a reduction in deformationof the anterior face of the post, as well as a reduction in the amountof bone that is removed during the surgery.

BACKGROUND

Joint replacement surgery is quite common and it enables manyindividuals to function normally when they otherwise would not bepossible to do so. Typically, an artificial joint includes metallic,ceramic and/or plastic components that are fixed to existing bone. Oneof the more common joints that undergoes replacement surgery is theknee. Knee arthroplasty is a well known surgical procedure by which adiseased and/or damaged natural knee joint is replaced with a prostheticknee joint. A typical knee prosthesis includes a femoral component, apatella component, a tibial tray or plateau and a tibial bearing insertcoupled to the tibial tray. The femoral component generally includes apair of laterally spaced apart condylar portions that have distalsurfaces that articulate with complementary condylar elements formed ina tibial bearing insert.

Total knee prostheses can essentially be classified into three basiccategories based on the techniques and components involved in thesurgery. In a first category, the articular surface of the distal femurand proximal tibia are “resurfaced” with respective condylar-typearticular bearing components. These knee prostheses provide substantialrotational and translational freedom and require minimal bone resectionto accommodate the components in the available joint space. Thepatella-femoral joint may also be resurfaced by a third prostheticcomponent, as well. The femoral, tibial and patella prostheticresurfacing components are affixed to respective adjacent bone structureby a cementing or by a biological bone ingrowth fixation means or anyother suitable technique.

The femoral component provides medial and lateral condylar bearingsurfaces of multi-radius design of similar shape and geometry as thenatural distal femur or femoral-side of the knee joint. The tibialcomponent can be made entirely of plastic (UHMWPE: ultra-high molecularweight polyethylene) or it can be made of a metallic base component andinterlocking plastic component. The plastic tibial bearing surface canbe of concave multi-radius geometry to more or less match the matingfemoral condyles. Both the femoral and tibial components areindependently positioned on either side of the knee joint and are notmechanically connected or linked together, as in the case of hinged typeof knee prostheses, which constitutes the secondary category of totalknee prostheses.

In resurfacing types of total knee prostheses according to the firstcategory, the tibial bearing surface geometry can assume a variety ofconfigurations, depending upon the desired extent of articular contactcongruency and associated translational (medial-lateral andanterior-posterior) and rotational (axial and varus-valgus) secondaryfemoro-tibial motions. These various secondary motions allow theresurfaced knee to function in a natural-like biomechanical manner inconjunction with the surrounding ligamentous and muscle structures aboutthe knee joint. The soft tissue structures maintain the femoral andtibial bearing surfaces in contact, provide the necessary levels offorce constraint to achieve knee joint stability, and functionallydecelerate the principal motion in flexion-extension and secondarymotions, such as axial rotation, in a controlled manner. Additionally,this functional interaction between the surrounding tissue structuresand the implanted knee prosthesis minimizes abrupt motion stoppage orimpact loading of the prosthetic articular surfaces, thus preventingoverstressing at the component fixation interface.

According to the second category, a mechanically linked, or hinged typeof knee prosthesis provides a fixed fulcrum flexion-extensioncapability. The “hinged knee” therefore is usually surgically indicatedin selected cases where the surrounding soft tissue structures aregrossly degenerated and incapable of providing functionally acceptableknee joint stability.

The third category of total knee prosthetic devices, the posteriorstabilized total knee provides more predictable kinematics than thefirst category. The posterior-stabilized total knee devices essentiallyincorporate all of the functional features of the first category, thatis, the resurfacing condylar-type of knee prostheses, in addition toincorporating a mechanical cam/follower mechanism for providingposterior (tibia-to-femur) constraint. The cam/follower mechanism ispositioned within the intercondylar space of the femoral component andprovides substitutional posterior constraint, as a predesignedcompensation feature for lost posterior cruciate function or forcompromised posterior knee stability. This cam/follower mechanismenables the femur to ‘roll-back’ on the tibia providing a mechanicaladvantage to the quadriceps during flexion.

The cam portion of the cam/follower mechanism, generally includes aconvex lobe shaped surface, integrally machined or cast within abox-like structure known as the “stabilizer box,” located between themedial and lateral condyle bearing surfaces of the femoral component asshown in FIG. 1. The stabilizer box can also be referred to as being anintercondylar portion of the femoral component. The cam surface isgenerally formed within the posterior wall portion of the stabilizer boxand is bounded by the superior wall on the top, the medial and lateralwall portions on the sides and the anterior portion. The stabilizer boxstructure, thus formed, occupies a significant envelope, relative to theoverall dimensions of the femoral component and therefore, requires asubstantial resection of viable bone to allow its accommodation withinthe intercondylar sector of the distal femur.

The posteriorly positioned articular convex surface of the cam isprecisely ground and highly polished. The convex cam articulates withthe anteriorly positioned and posteriorly oriented follower, as the kneeundergoes femoro-tibial flexion. The mating follower surface istypically machined integral within the ultra-high molecular weightpolyethylene (UHMWPE) tibial component. The follower member usuallyconsists of a relatively convex or flat articular surface located on theposterior side of an upwardly extending post-like structure, which ispositioned between the concave medial and lateral tibial plateau bearingsurfaces. The resultant action of the contacting cam/follower mechanismprovides posterior stabilization or constraint of the tibial component,relative to the femoral component: generally from about mid-range tofull range of flexion. Within this limited range, therefore, thestabilizing mechanism essentially simulates the functional contributionof the natural posterior cruciate ligaments attached between theanterior femur and posterior tibia aspects of the knee joint.Additionally, since the cam/follower surface geometry is generallynon-congruent, the mechanism can be designed to produce posteriorroll-back of the femoro-tibial articular contact, simulating the naturalbiomechanical displacement characteristics of the natural knee.

Examples of posterior-stabilized total knee prostheses of the typedescribed above, are disclosed in U.S. Pat. Nos. 4,209,861 to Walker;4,298,992 to Burstein et al.; 4,213,209 to Insall et al.; and 4,888,021to Forte et al. Each of the devices described in the above patentsincorporates a UHMWPE tibial component with a pair of medial and lateralconcave plateau bearing surfaces and a metal alloy femoral componentwith mating multi-radius condylar runners which ride on the bearingsurfaces. The articulation of the femoral condyles with the tibialplateau bearing surfaces allows primary femoro-tibial flexion andextension, and secondary (freedom) motions of axial and varus-valgusrotations and anterior-posterior and medial-lateral translations. Theknee joint reaction forces during primary or secondary motion areprincipally supported by the tibial bearing surfaces, and to some extentby the cam/follower surfaces, and are transferred to the underlyingfixation interfaces and adjacent supportive bone structures.

Additionally, the UHMWPE tibial component incorporates an upwardlyextending post-like structure which is positioned between the plateaubearing surfaces, slightly anterior of the component mid-line. Thegenerally convex or flat follower surface is integrally machined on theposterior-side of the post. With the femoral and tibial knee componentsin a normally reduced, surgically implanted position, the upwardlyextending tibial post extends into the stabilizer box structure locatedwithin the intercondylar space of the femoral component. Posteriortibial constraint is achieved when the posteriorly oriented face of thefollower contacts the generally anteriorly oriented lobe surface of thecam.

However, there are a number of disadvantages with the geometries ofconventional posterior cruciate substituting knee designs. Inparticular, one common complaint among knee surgeons is that posteriorcruciate substituting knee replacements remove too much bone. Excessivebone removal can lead to intraoperative intercondylar fractures due tothe stress concentration created by cutting out bone to accommodate thebox of the design. Bone removal is also not desired in that in the eventof revision surgery, the more bone available, the easier the revisionsurgery will be. It is therefore desirable and there is a need for animproved posterior cruciate substituting knee design that minimizes theamount of bone that is needed to be removed.

Another limitation with conventional posterior cruciate substitutingknee designs is that the retrieved knee replacements show consistentdeformation patterns in particular locations on the central post of thetibial insert that is typically made from UHMWPE. A common location forthe damage to the tibial insert is the anterior face of the post. Thisdeformation is often in the form of a “bowtie” pattern and is the resultof the continued interaction of the implant components over time andlikely occurs when the patient hyperextends their knee. In rare cases,this deformation can contribute to gross mechanical failure of the post.In view of the foregoing, there is a need for an improved posteriorcruciate substituting knee design that reduces the stresses thatcontribute to this pattern of deformation.

SUMMARY

According to one aspect of the present invention, a tibial component fora knee joint prosthesis includes a platform having an upper surface thatincludes first and second laterally spaced concavities. Each concavityis adapted for receiving one condylar portion of a femoral component.The upper surface also includes a tibial post that fits within theintercondylar space of the femoral component. The tibial post has ananterior cam surface that has a saddle shaped surface (“saddle surface”)which is a smooth surface that derives is name from the peculiar shapeof historical horse saddles, which curve both up and down as describedin more detail. The cam surface includes a saddle shaped portion that islocated at an inferior part of the anterior cam surface and a transitionportion that is located at a superior part of the anterior cam surface.The functional part of the anterior post is defined by the saddle shapedportion, while the convex transition portion is provided and shaped toblend the anterior portion of the cam to the top of the post.

In another aspect, a knee joint prosthesis includes a femoral componenthaving an anterior side and a posterior side and including a pair oflaterally spaced condylar portions and an intercondylar portion joiningthe condylar portions and including a recess. The prosthesis furtherincludes a cam surface located adjacent the intercondylar recess on theanterior side of the femoral component, with the cam surface beingdefined by an at least substantially concave first radius of curvatureand an at least substantially convex third radius of curvatureperpendicular to the first radius of curvature so as to create a saddletype shape.

The tibial post has an anterior cam surface that is defined by an atleast substantially convex second radius of curvature and an at leastsubstantially concave fourth radius perpendicular to the second radiusof curvature so as to define a saddle shaped anterior cam surface thatis complementary to the saddle shaped anterior cam surface of thefemoral component. The second radius of curvature is less than the firstradius of curvature. According to one embodiment, the second radius ofcurvature is equal to or less than 95% of the first radius of curvature.The third radius of curvature is approximately 42% or less of the fourthradius of curvature.

In another aspect, the intercondylar portion of the femoral componentincludes a roof and has a box angle of greater than 20 degrees (e.g., 28degrees) as measured from the roof to a plane parallel to a base(ground) plane.

By modifying the anterior cam surface of the femoral component and bymodifying the anterior face of the stabilizing post of the tibialinsert, a reduction of stresses (von Mises stresses in the tibial post)at these surfaces and reduced deformation of the anterior face of thetibial post are realized.

Further aspects and features of the exemplary joint prosthesis disclosedherein can be appreciated from the appended Figures and accompanyingwritten description.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is side perspective view of a conventional femoral component thatforms a part of a knee joint prosthesis;

FIG. 2 is a side perspective view of a femoral component according toone embodiment of the present invention that forms a part of a kneejoint prosthesis;

FIG. 3 is a side elevation view of the femoral component of FIG. 2;

FIG. 4 is a bottom partial plan view of a cam surface of the femoralcomponent of FIG. 2;

FIG. 5 is a top perspective view of the femoral component of FIG. 2;

FIG. 6 is a side elevation view showing an increase in box angle in thefemoral component of FIG. 2 compared to the component of FIG. 1;

FIG. 7 is a side perspective view of a tibial component according to oneembodiment of the present invention;

FIG. 8 is a side elevation view of the tibial insert of FIG. 7;

FIG. 9 is side elevation view of the femoral component of FIG. 2 matedwith the tibial component of FIG. 7 in approximately 10 degrees ofhyperextension;

FIG. 10 is a cross-sectional view taken along the line 10-10 of FIG. 9;

FIG. 11 is a side elevation view showing the femoral bone cuts toreceive the conventional femoral component and the femoral component ofthe present invention; and

FIG. 12 is cross-sectional view of an exemplary anterior cam surface ofeither the tibial post or femoral component illustrating a flat formedalong the arcuate surface thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 2-10 illustrate a joint prosthesis, in the form of a knee jointprosthesis 100 (FIG. 9), according to one exemplary embodiment of thepresent invention. The illustrated prosthesis 100 is of a posteriorcruciate substituting knee design. The knee relies on four mainligaments to provide stability and support. There are two ligaments thatcross in the center of the knee, and they are called the cruciateligaments. The anterior cruciate ligament (ACL) prevents the femur fromcoming off the back of the tibia. The posterior cruciate ligament (PCL)prevents the femur from coming off the front of the tibia. Posteriorstabilized knee implants are designed to be a substitute for theposterior cruciate ligament. As described in detail below, a posteriorstabilized knee includes a feature, such as a post, that substitutes forthe body's posterior cruciate ligament.

The prosthesis 100 generally includes a femoral component 110 (FIG. 2)for attachment to the femur and a tibial component 200 (FIG. 8) forattachment to the tibia. The femoral component 110 is formed of a body112 that has a pair of laterally spaced-apart femoral condylar portions114, 116, each of which is smoothly convexly curved in a lateral profilegenerally to approximate the curvature of an anatomical femoral condyleand is convexly curved along its antero-posterior extent. The anteriorparts of the condylar portions merge smoothly with convexly curvedlateral portions 122 of a patellar portion 120. A midportion 126 of thepatellar portion 120 intersects at its inferior extremity a superiorwall or roof 132 of a box-like intercondylar portion 130 (stabilizerbox), which together with the patellar portion 120, connects thecondylar portions 114, 116.

As shown in FIG. 1, the intercondylar portion of the conventionalfemoral component 10 is a rectangular-shaped box defined by a pair oflaterally spaced-apart side walls that are joined by a flatperpendicular roof as shown in the side view of FIG. 1. A box angle ofthe intercondylar portion is about 8.3° as measured from the roof of theintercondylar portion 130 to a horizontal plane (parallel to nominalbase plane) as shown in FIG. 6.

In contrast to the rectangular box shape of the intercondylar portion ofthe prior art femoral component 10, the design of the intercondylarportion 130 of the present invention has been modified so that theamount of bone that has to be removed is reduced. As previouslymentioned, one disadvantage of the prior art implant designs is thatposterior cruciate substituting knee replacement techniques remove toomuch bone and this excessive bone removal can lead to intraoperativeintercondylar fractures due to the stress concentration created bycutting out bone to receive the box shaped intercondylar portion 130.FIG. 11 shows a section of the femur bone that has the bone removed tofit the intercondylar portion of the conventional femoral component 10shown in FIG. 1. As would be expected, in order to accommodate therectangular-shaped box of the conventional femoral component, arectangular shaped bone segment is removed from the femur so as to leavea rectangular shaped notch or opening in the femur. In contrast, FIG. 11shows an arcuate shaped bone segment removed from the femur so as toleave an arcuate shaped notch or opening in the femur to accommodate thedevice 100 of the present invention.

According to one exemplary embodiment, the modification in the boxgeometry according to the present invention results in an averagereduction of about 37% in the volume of the bone removed from theintercondylar portion of the femur. This reduction in bone removal isaccomplished by two means. First, the angle of the intercondylar portion130 of the femoral component 110 is increased to minimize the boneremoved anteriorly as shown in FIG. 6. For example, the box angle, asmeasured from a top of the roof 132, is significantly increased relativeto the box angle of the conventional intercondylar portion and in theembodiment illustrated in FIGS. 2-6, the box angle is increased from8.3° (conventional design) to about 28°. However, it will be appreciatedthat the above values are not limiting but are merely exemplary innature and therefore, the box angle can be about 20 degrees to about 35degrees, e.g., 29 degrees to about 34 degrees. In other embodiments, thebox angle is increased to a value that is at least twice the value of asimilar rectangular-shaped intercondylar box construction. The increasedheight of the posterior box does not remove much bone as there isminimal bone in this region of the femur.

The second means for reducing the amount of bone removal is themodification of the intercondylar box from a squared off configurationto more of a cylindrical shape as shown in FIG. 2, thereby removing lessbone at the corners of the box. The intercondylar portion 130 is definedby an arcuate shaped wall 131 that likewise defines the roof 132 of theportion 130. The roof 132 can thus be thought of as the apex region ofthe arcuate shaped wall 131. The illustrated arcuate shaped wall 131 hasa semi-circular shape or “rounded shape” that is designed to be receivedwithin a complementary rounded bone notch or opening that is shown inFIG. 11. The present intercondylar design thus does not include a welldefined roof that is generally horizontal (parallel to a nominal baseplane).

A comparison of the figures in FIG. 11 shows that significantly lessbone is removed in the design of the present invention since the hardsquared edges of the conventional femur box notch are absent in therounded femur box notch made according to the present invention.

The cylindrical shape of the femur box notch made in the femur can becut with a rotating cutter, such as a drill or reamer, which eliminatesthe additional stress concentrations created by the overcut slots thatare created when cutting the box geometry of FIG. 11 with a sagittalsaw. In other words, the cylindrical box geometry can be cut withoutcreating stress concentrations in the corners where a sagittal saw wouldextend the cut past the edge of the box.

An opening 160 is preferably formed in the roof 132 of the intercondylarportion 130 and in particular, the opening 160 is formed in the arcuateshaped wall 131. Since the roof in the prior art intercondylar portionis a flat, planar surface, the opening was contained in the same plane;however, the arcuate shape of the wall 131 causes the opening 160 to lienot in a single plane, but instead, the opening 160 lies in an arcuateshaped surface. The opening 160 allows for placement of anintramedullary nail in the event of a distal femoral fracture aftertotal knee replacement.

As best shown in FIG. 4, an underside of the femoral component 110includes an arcuate surface 170 (e.g., a curved saddle shaped surface).This arcuate surface 170 is located adjacent the opening 160 and facesthe tibial component 200 (FIG. 8) when the two components 110, 200 areassembled. The arcuate surface 170 is proximate the patella portion 120.According to the present invention, this arcuate surface 170 isconfigured and dimensioned so as to mate with a complementary surface ofthe tibial component 200 when the components 110, 200 mate together asdescribed below.

The femoral component 110 also includes a cam follower surface 180 thatis located adjacent the opening 160 at the posterior side of the femoralcomponent 110. In particular, the cam follower surface 180 is positionedbetween the condylar portions 114, 116. From the underside of theintercondylar portion 130, the cam follower surface 180 has a curvedsurface 182 that merges with a substantially concave portion 184 thatthen curves inward at 186 to merge with an upper curved surface 188.

The femoral component 110 can be made of a number of differentmaterials, including a surgical grade, durable metal, such as a 316Lstainless steel or a chrome-cobalt-molybdenum meeting ASTM Standard#F75. All surfaces which are external to the bone are preferably highlypolished and the femoral component 110 can be symmetrical about avertical antero-posterior center plane so that it can be used on eitherknee. It also can be asymmetrical (i.e., right or left knee specific).

The surfaces of the femoral component 110 that face the femur aregenerally flat and each of the condylar portions 114, 116, can bebounded by a small rib or flange, thus to provide a dam to increasecement pressurization and simplify clean up of excess cement. Thispocketed feature also allows for beads or other biological attachmentsurfaces.

The tibial component 200 includes a tibial platform or tray 210 fromwhich a tibial stem 212 extends downwardly and is constructed forinsertion and attachment to the tibia. An upper surface 214 of thetibial tray 210 is constructed to receive and attach to a bearingcomponent (tibial insert) 220 that is positionable between the femoralcomponent 110 and the tibial tray 210. As described in greater detailbelow, the tibial insert 220 cooperates with the femoral component 110to provide for the desired kinematics of the knee prosthesis.

The tibial insert 220 of the tibial component 200 is typically formed ofa suitable plastic such as polyethylene, and more particularly, UHMWPE;however, other suitable materials can be used so long as they areintended for use in the current application. As shown best in FIGS. 7-9,the tibial insert 220 includes an oblong, rounded, disc-like plateauportion 222 having an upper surface that can be flat or have some otherpredetermined contour. A pair of laterally spaced-apart, oblongconcavities 224, 226 is formed along the upper surface for receivingfemoral condylar portions 114, 116 of the femoral component 110. The“nested” support of the femoral component 110 stabilizes the prostheticjoint, but still permits antero-posterior translation, lateralangulation and rotation, all of which are involved in normal function ofthe anatomical knee joint.

The tibial insert 220 also includes a base-like fixation portion 230that extends from a bottom surface 228 of the plateau portion 222 toallow the tibial insert 220 to be attached to the tibial tray 210 usingconventional techniques and methods.

The tibial insert 220 also includes a stabilizing post 240 that extendsupward from the plateau portion 222 between the concavities 224, 226 andis positioned to be received in an intercondylar recess of the femoralcomponent 110. The stabilizing post 240 is generally triangular in alateral profile and is defined by flat, parallel side surfaces 242, ananterior face 250, and an opposite posterior face 260. The side surfaces242 of the stabilizing post 240 are in sufficient clearance from thelateral walls of the femoral intercondylar recess to allow for normallateral angulation and rotation when assembled with the femoralcomponent 110 of the prosthetic knee joint. The posterior face 260 ofthe stabilizing post 240 includes a concave surface 262 at the inferiorpart of the posterior face 260 and furthermore, the posterior face 260has a superior posterior surface 261 portion.

In contrast to conventional implants that have flat anterior faces, theanterior face 250 of the present invention does not have a flat designbut instead, the anterior face 250 has been modified and constructed tocreate a lower stress contact condition when the patient hyperextendstheir knee. The anterior face 250 of the post 240 has a curved sweptsurface that takes the form of a saddle-like configuration where an atleast substantially convex curve is swept along an at leastsubstantially concave curve to form a saddle shape (i.e., this portionof the cam surface curves up in one or more directions and curves downin one or more directions).

A saddle shaped surface can be expressed in terms of saddle points. Asaddle point for a smooth function, such as a curve or surface, is apoint such that the curve/surface in the neighborhood of this point lieson different sides of the tangent at this point. The surface at a saddlepoint resembles a saddle that curves up in one or more directions, andcurves down in one or more other directions (similar to a mountainpass). In terms of contour lines, a saddle point can be recognized, ingeneral, by a contour that appears to intersect itself.

Thus, the anterior face 250 has a generally convex shape in a lateral ortransverse direction, while in a longitudinal direction (from theinferior part to the superior part) the anterior face 250 transitionsfrom a concave portion 252 at the inferior part of the anterior face 250to a convex portion 254 at a superior part of the anterior face 250, asobserved in a longitudinal direction and as shown in FIG. 8. In otherwords, the swept curved nature of the anterior face 250 is defined by atransition in the longitudinal direction from the concave portion 252 atthe base of the post 240 to the convex portion 254 at the top of thepost 240 (while at the same time, the anterior face 250 has a convexshape in a transverse direction (side-to-side direction perpendicular tothe longitudinal direction) from the inferior part to the superior partso as to create the saddle configuration). It will be appreciated thatthe shape of the portion 254 at the top of the post is not criticalsince its illustrated convex shape simply provides a smooth way toconnect the concave portion of the top of the post.

According to the present invention, the radius of curvature of theanterior face 250 is selected in view of a complementary radiusassociated with the femoral component 110. In particular, the radii areselected so that they are not identical, but instead, there is a slightmismatch in the radii where the radius on the tibial component 200(i.e., the tibial post 240) is less than the femoral radius. In otherwords, the exact sizes of the radii are not critical so long as theradius of curvature of the anterior face 250 is a predeterminedpercentage of the femoral radius that results in a mismatch between theradii and the components 110, 200 to help assure that the contactbetween the components 110, 200 occurs at the center of the tibial post240 instead of at lateral edges of the post 240. In one embodiment, theradius on the tibial component 240, and in particular on the anteriorface 250, is approximately 95% of the femoral radius which is measuredalong the arcuate surface 170 (e.g., a curved saddle shaped surface thatis complementary to the curved saddle shaped surface of the tibial post240). FIG. 10 shows a transverse cross-section to illustrate the matingof the two saddle shaped surfaces, one associated with the femoralcomponent 110 and the other with the tibial component 200.

It will be appreciated that the present invention is directed toimprovements and modifications to the anterior cam (surface 170) on thefemoral component 110 and the anterior cam (surface 250) on the tibialcomponent 200. It will be appreciated that due to their saddle shapedconstructions, both surfaces 170 and 250 are described as being camsurfaces that are configured to engage with one another similar to how atraditional cam and cam follower engage one another.

As discussed above and in accordance with the present invention and asbest shown in FIGS. 9 and 10, there is a relationship between the radiusof curvature of the anterior face 250 and the radius of curvature of thecomplementary femoral component 110, and more particularly, the arcuatesurface 170 formed on an underside thereof adjacent the opening 160. Theradii of curvature of the surfaces/faces 170, 250 are not identical, butrather, there is a slight mismatch between the two in that the radius ofcurvature of the anterior face 250 is less than the radius of curvatureof the surface 170. In one exemplary embodiment, the radius of curvatureof the anterior face 250 is equal to or less than 95% of the femoralradius (i.e., the radius of curvature of arcuate anterior surface 170).For example, in one embodiment, the radius of the anterior face 250 ofthe post 240 is about 9.5 mm, while the radius of curvature of thecomplementary anterior face (surface) 170 of the femoral component 110is about 10.0 mm (ratio of 95%). However, other radii are equallypossible for the components 110, 200 and the radius on the tibialcomponent 200 can be less than 95%, and even less than 90%, of thefemoral radius.

Surface deformation in the anterior face of the post of the tibialcomponent should not necessarily be expected; however, it can be causedby surgical malpositioning of the femoral and tibial components or bythe designs of the components themselves or if a patient excessivelyhyperextends their knee. Retrievals of posterior stabilized total kneeimplants consistently show deformation patterns in this anterior faceregion. Implanting the femoral component in flexion or the tibialcomponent tilted posteriorly may cause premature hyperextension contact.Conventional posterior stabilized implants, such as the one illustratedin FIG. 1) were not specifically designed to reduce the stresses on theanterior face, because patients were not expected to utilize theanterior surface of the post as a hyperextension stop.

This behavior between the components was modeled using finite elementanalysis (FEA). The stress state contributing to the pattern ofdeformation on the tibial post 240 was described and the effects of theabove modifications that were made to the post-cam design for reducingthe stresses on the anterior face 250 of the post 240 were examined.

EXAMPLE

Computer models of the conventional implant of FIG. 1 (i.e., theExactech Optetrak® PS total knee prosthesis) were modified to facilitatefinite element meshing of the tibial post and the femoral anterior cam.The components were positioned in 10° of hyperextension. In thisexample, the anterior cam of the femoral component was modeled as arigid indenter. The post of the tibial component was modeled as UHMWPEusing a true stress-strain relationship. The constitutive model for thismaterial was based on a von Mises yield surface with isotropichardening. FE meshes were created, with the tibial post FE mesh beingconstructed using 8-noded hexagonal brick elements and the anterior camsurface being composed of 4-noded rectangular rigid elements. Becausethe post-cam mechanism is symmetric about the sagittal plane, asymmetric boundary condition was used and only half the mechanismmodeled. The distal face of the post was fixed in all directions and thecam was allowed translation only in the direction of contact, i.e.,perpendicular to the post at the contact point.

A load of 445N was used based on a 2D free body diagram of loads derivedfrom gait data at maximum hyperextension. This load was applied to therigid cam indenter, and its direction was perpendicular to the post atthe point of contact. Analyses were carried out using three differentsizes of the conventional implant of FIG. 1 and the modified designaccording to the present invention that is shown in FIGS. 2-10.

In all three sizes of the conventional implant, the maximum von Misesstress was located at the lateral edge of the anterior face of tibialpost slightly inferior to the line-to-line contact point of the post andcam. The magnitudes were 34 MPa for size 2, 37 MPa for size 3 and 42 MPafor size 4 (all conventional designs). Maximum deformation of the UHMWPEpost occurred at the same location as maximum stress and also increasedwith implant size; the values were 0.23 mm; 0.27 mm; and 0.36 mm forsizes 2, 3, and 4, respectively.

By modifying the design of the contact surfaces (e.g., anterior face 250and surface 170), maximum von Mises stress decreased 35% to 24 MPa andmaximum displacement decreased 37% to 0.17 mm compared to the size 3conventional implant.

Stress contours in the FE models qualitatively matched the deformationpattern observed on retrieved implants. Maximum von Mises stressoccurred on the lateral edge of the anterior tibial face, where contactwas initiated. Stress was high in this region because the femoral camindents the lateral edge before line-to-line contact occurs across thewidth of the face. Stresses increased with implant size because lateraledge indentation increased with size as the distance that the femoralcam must travel to reach line-to-line contact increased from size 2 tosize 4. Contact in the new implant design of FIGS. 2-10 was initiated atthe center of the post, eliminating lateral edge loading. The changes inthe contact surfaces also broadened the contact area in theproximal-distal direction leading to a wider stress distribution.

In this manner, damage to the post 240 is reduced by modifying theshapes of the femoral and tibial components 110, 200 to reduce thecontact stresses in the post. According to the present invention, theshape of the anterior surface 170 of the femoral component 110 and theanterior face 250 of the post 240 are modified to create a lower stresscontact condition when the patient hyperextends their knee. The mismatchin the radii of curvatures between the two complementary mating surfaces170, 250 assures that the contact between the components 110, 200 occursat the center of the tibial post 240 instead of the lateral edges of thepost 240.

The present design thus offers a more robust design with less contactsurface stresses and less deformation of the anterior face of thestabilizing post that is part of the tibial insert.

As shown in FIG. 12, it will be appreciated that, in one embodiment, theanterior arcuate cam surfaces of each of the tibial post and theintercondylar portion of the femoral component can include a flat formedalong the radius of curvature. In the case of the tibial post, the flatis formed along the convex anterior cam surface and in the case of theintercondylar portion, the flat is formed along the concave anterior camsurface. The width of the flat is relatively small and does not impactthe above described mismatch in the radii of curvature of the twocomponents. The flats should be positioned along their respectiveanterior cam surfaces so that they contact one another when the tibialand femoral components mate with one another. As illustrated, the flatis typically formed in a central area of each respective anterior camsurface. The anterior cam surface of the tibial component is thus atleast substantially convex in that can include a small flat formed alongits radius of curvature and the anterior cam surface of the femoralcomponent is thus at least substantially concave in that it can includea small flat formed along its radius of curvature.

However, in other embodiment, as shown in FIGS. 2-11, the flats can beeliminated.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to the embodiments described thus far withreference to the accompanying drawings; rather the present invention islimited only by the following claims.

1. A knee joint prosthesis comprising: a femoral component having ananterior side, a posterior side, a pair of laterally spaced condylarportions, and an intercondylar portion joining the condylar portions andincluding a recess; a cam surface located adjacent the intercondylarrecess on the anterior side of the femoral component, the cam surfacehaving a saddle shape and being defined by an at least substantiallyconcave first radius of curvature and a convex third radius of curvaturethat is perpendicular to the concave first radius of curvature; and atibial component including a platform having an upper surface thatincludes first and second laterally spaced concavities, each adapted forreceiving one condylar portion of the femoral component and a tibialpost for reception in the intercondylar recess of the femoral component,the tibial post having a saddle shaped anterior cam surface that iscomplementary to the saddle shaped anterior cam surface of the femoralcomponent and is defined by an at least substantially convex secondradius of curvature and a concave fourth radius of curvature that isperpendicular to the convex second radius of curvature; wherein theconvex second radius of curvature of the tibial component is less thanthe concave first radius of curvature of the femoral component; whereinthe saddle shaped anterior cam surface of the tibial post mates with thesaddle shaped cam surface of the femoral component in an articulatingmanner in approximately 10 degrees of hyperextension of the knee.
 2. Theknee joint prosthesis of claim 1, wherein the convex second radius ofcurvature is 95% or less of the concave first radius of curvature. 3.The knee joint prosthesis of claim 1, wherein the convex second radiusof curvature is 90% or less of the concave first radius of curvature. 4.The knee joint prosthesis of claim 1, wherein the convex third radius ofcurvature is between about 29% to about 42% of the concave fourth radiusof curvature.
 5. The knee joint prosthesis of claim 1, wherein theintercondylar portion includes an arcuate shaped roof and has a boxangle of greater than 20° as measured from the roof to a plane parallelto a nominal base plane.
 6. The knee joint prosthesis of claim 1,wherein the saddle shaped anterior cam surface of the tibial post isformed at an inferior part thereof.
 7. The knee joint prosthesis ofclaim 1, wherein each of the spaced condylar portions has a surface thatis convexly curved antero-posteriorly to match generally a lateralprofile of an anatomical femoral condyle and convexly curved laterallythroughout its antero-posterior extent.
 8. The knee joint prosthesis ofclaim 1, wherein each of the at least substantially convex second radiusof curvature of the anterior cam surface of the tibial post and the atleast substantially concave first radius of curvature of theintercondylar portion of the femoral component includes a flat formedalong the radius, each flat being positioned so that they contact oneanother when the two components mate with one another.
 9. The knee jointprosthesis of claim 1, wherein the intercondylar portion has an arcuateshaped roof and an angle of the intercondylar portion, as measured fromthe roof, to a horizontal plane, which is parallel to a base (ground)plane, is greater than 25° .
 10. The knee joint prosthesis of claim 9,wherein the arcuate shaped roof is configured to be inserted into anarcuate shaped notch formed in the femur by removing bone.
 11. The kneejoint prosthesis of claim 9, wherein the angle of the intercondylarportion is between about 20° to about 35°.
 12. A knee joint prosthesiscomprising: a femoral component having an anterior side, a posteriorside, a pair of laterally spaced condylar portions, and an intercondylarportion joining the condylar portions and including a recess; a camsurface located adjacent the intercondylar recess on the anterior sideof the femoral component, the cam surface being defined by an at leastsubstantially concave first radius of curvature; and a tibial componentincluding a platform having an upper surface that includes first andsecond laterally spaced concavities, each adapted for receiving onecondylar portion of the femoral component and a tibial post forreception in the intercondylar recess of the femoral component, thetibial post having an anterior cam surface defined by an at leastsubstantially convex second radius of curvature; wherein the first andsecond radiuses of curvature are different from one another to create amismatch between the radii to encourage that the contact between thetibial and femoral component occurs at a center region of the tibialpost instead of at lateral edges thereof; wherein the intercondylarportion includes an arcuate shaped roof that extends between a pair oftangent side walls and has a box angle of greater than 20° as measuredfrom the roof to a plane parallel to a nominal base plane, theintercondylar portion being configured so as to permit reception intoand mating with a femur box notch formed in a femur bone, the femur boxnotch being defined by a rounded top surface and a pair of parallel sidewalls so as to permit the arcuate shaped roof to seat against therounded top surface and the side walls of the intercondylar portion toseat against the side walls of the femur box notch.
 13. A knee jointprosthesis comprising: a femoral component having an anterior side, aposterior side, a pair of laterally spaced condylar portions, and anintercondylar portion joining the condylar portions and including arecess, the intercondylar portion having an arcuate shaped roof; a camsurface located adjacent the intercondylar recess on the anterior sideof the femoral component, the cam surface having a saddle shape andbeing defined by an at least substantially concave first radius ofcurvature and a convex third radius of curvature that is perpendicularto the concave first radius of curvature; and a tibial componentincluding a platform having an upper surface that includes first andsecond laterally spaced concavities, each adapted for receiving onecondylar portion of the femoral component and a tibial post forreception in the intercondylar recess of the femoral component, thetibial post having a saddle shaped anterior cam surface that iscomplementary to the saddle shaped anterior cam surface of the femoralcomponent and is defined by an at least substantially convex secondradius of curvature and a concave fourth radius of curvature that isperpendicular to the convex second radius of curvature; wherein theconvex second radius of curvature of the tibial component is less thanthe concave first radius of curvature of the femoral component; whereinthe saddle shaped anterior cam surface extends to an inferior part ofthe cam surface of the tibial post and is in articulating contact withthe saddle shaped cam surface of the femoral component, the arcuateshaped roof of the intercondylar portion being configured to be insertedinto an arcuate shaped notch formed in the femur by removing bone, withthe intercondylar portion being received between and placed in contactwith two parallel side walls that define the acuate shaped notch.